Laminated films with a hard coat layer provided over the film surface have been widely used in manufacturing antireflection film, film for touch panels, and film for face plates.
In recent years, markets have been rapidly expanding for flat panel displays (FPD) such as liquid crystal displays (LCD), plasma displays (PDP), and organic electroluminescent (organic EL) displays, and there are increasing demands for products with a larger screen and high definition. Accordingly, development of components with highly advanced functions has been strongly called for to serve in manufacturing these products.
In such circumstances, most of the conventional antireflection films to reduce the surface reflection on these displays consist of a base film of triacetate or polyester coated with a hard coat layer and further covered with a high refractive index layer and then a low refractive index layer so that interface reflections between these layers are cancelled to reduce the overall reflection.
However, antireflection films with a lower reflectance, smaller thickness, and lower price have been required more strongly, and accordingly, and increased efforts have recently been made toward developing novel antireflection films consisting of a polyester film directly coated with a hard coat layer with a high refractive index and further covered with a low reflectance layer. Antireflection films of such a structure, however, tend to suffer interference patterns caused by insufficient adhesion, difference in refractive index between the film and the hard coat layer, and uneven thickness, leading to visibility problems. If an adhesion layer is formed over the polyester film with the aim of improving the adhesive property, improved adhesive property will actually be achieved, but such an adhesion layer generally has a lower refractive index that the polyester film used as base material, and consequently the difference in refractive index between the adhesion layer and the hard coat layer, which has a high refractive index, will further increase. Thus, there remains the difficulty in eliminating the interference pattern.
To control the interference pattern that develops when a hard coat layer with a high refractive index is formed over the polyester film, a study has proposed a method in which monomers containing an aromatic substituent group, such as fluorene group, are copolymerized with the resin of the adhesion layer to increase the refractive index of the resin itself (Japanese Unexamined Patent Publication (Kokai) No. HEI-10-110091). However, when using a so-called in-line coating process in which a polyester film that is not fully crystal-oriented is subjected to corona discharge treatment as needed, followed by coating with an adhesive coating material, drying, stretching, and heat treatment to complete the crystalline orientation, a high refractive index resin commonly has a rigid chemical structure although a water dispersion of the highly refractive resin is necessary. A highly hydrophilic sulfonate group has to be used in large amounts to disperse water in the resin, leading to poor adhesion under high temperature and high humidity conditions. Furthermore, a resin copolymerized with a fluorene group generally has a high glass transition temperature and cannot be stretched smoothly, leading to poor spreadability in an in-line coating process and fine cracks in the adhesion layer to cause undesired haze in the film.
Another study focuses on developing an adhesion layer containing highly refractive fine particles of metal oxide, such as titanium oxide, to prepare a polyester film provided with an adhesion layer with an increased refractive index (Japanese Unexamined Patent Publication (Kokai) No. 2004-54161). If it is applied to a common in-line process, however, problems tend to take place such as surface scattering by particle projections and formation of voids between coagulated particles or at the interface between particles and the binder interface, causing undesired haze in the film.
Another study focuses on developing an adhesion layer containing a water-soluble titanium chelate compound or zirconium compound to prepare a polyester film provided with an adhesion layer with an increased refractive index (Japanese Unexamined Patent Publication (Kokai) No. 2005-97571). However, such chelate compounds generally contain only small quantities of titanium and zirconium, and large amounts of these chelate compounds have to be added to improve the refractive index. These metal chelate compounds can decompose during heat treatment, and the decomposition products will form undesired material in the adhesion layer under some conditions, leading to an antireflection film with decreased quality.
It could therefore be helpful to provide an optical-purpose adhesion film that serves to reduce the interference pattern and develops good adhesion to the hard coat layer when used as base of an antireflection film. It could also be helpful to provide a laminated polyester film that has good characteristics including high refractive index, high strength, and high heat resistance, shows good adhesion under high temperature and high humidity conditions, and achieves a high-level spreadability in in-line coating processes, in addition to having the ability to reduce the interference pattern.